The present invention relates to an image forming method for controlling the rotations of photosensitive drums in both a black mode and a color mode in a tandem-drum type color image forming apparatus.
Prior-art techniques, as described below, are disclosed as regards a rotation control of a plurality of photosensitive drums in a tandem-drum type color image forming apparatus.
Jpn. Pat. Appln. KOKAI Publication No. 2000-284569 discloses an image forming apparatus wherein a set of registration patterns of respective colors are formed on a transfer belt by repeating them twice or more within a time period that is not less than a cycle of the drum and is less than a circulation cycle of the transfer belt. The registration patterns are detected, and information on the amount of a color registration error, or a color misregistration, of the respective colors of each set is acquired. A color registration error component of the drum rotation cycle and a component of the transfer belt circulation cycle are extracted from the acquired information, and the timing of writing of each scan line on the drum is corrected so as to eliminate a color misregistration on the basis of the extracted components.
This image forming apparatus includes phase detection means for detecting a variation in rotational phase of the respective drums. When this means detects a rotational phase difference, a registration pattern corresponding to the length of one cycle of the drum is formed, and the data on the color registration error component of the drum rotation cycle is updated.
Jpn. Pat. Appln. KOKAI Publication No. 11-119502 discloses an image forming apparatus having a plurality of drums and a single drive means for rotating the drums, wherein the rotations of the drums are synchronized by a rigid coupling member that couples each drum and the drive means.
Jpn. Pat. Appln. KOKAI Publication No. 10-339984 discloses a tandem-type image forming apparatus wherein a convey belt is driven by a drive roller, and while drums are driven by the convey belt, an image formed on each drum is successively transferred onto a transfer medium.
In this image forming apparatus, the peripheral speed of the drum and the peripheral speed of the convey belt are measured, and the convey belt peripheral speed is subtracted from the measured drum peripheral speed to calculate a correction drum peripheral speed. Based on the calculated correction drum peripheral speed, the scan operation of image write means is controlled so as to correct a registration error of the image write position due to a variation in the drum peripheral speed.
The problems with these image forming apparatuses will now be discussed.
In Jpn. Pat. Appln. KOKAI Publication No. 2000-284569, a rotational phase difference is always detected after a mode, such as a black mode, in which the transfer belt and only a specified drum are rotated. In each of such cases, a registration pattern corresponding to the length of one cycle of the drum needs to be formed and the data on the color registration error component of the drum rotation cycle has to be updated. Consequently, when an image including a black mode component is output, the productivity considerably deteriorates (the time needed up to completion of output increases). Moreover, the life of drums, developer, etc. becomes shorter.
In Jpn. Pat. Appln. KOKAI Publication No. 11-119502, the drums are coupled by the rigid coupling member and synchronized. Thus, it is not possible to output an image by rotating only a specified drum as in a black mode. Even when an image including only a black page is to be output, all the drums and developing devices are rotated and the life of the drums for color images and developing devices decreases.
Jpn. Pat. Appln. KOKAI Publication No. 10-339984, sensors need to be provided for measuring the peripheral speeds of the drums and belt drive roller. In addition, a memory area for storing calculated correction drum peripheral speed, as well as complex arithmetic operations, is required. Further, before and after a mode, like a black mode, in which the transfer belt and only a specified drum are rotated, the phase of drums, which have been at rest, and the phase of the transfer belt are displaced. In each of such cases, the correction drum peripheral speed has to be calculated. As a result, when an image including a black mode component is output, the productivity considerably deteriorates (the time needed up to completion of output increases). Moreover, the life of drums, developer, etc. becomes shorter.
As has been described above, before and after the black mode (in which only the black drum rotates and the other color drums are set at rest), the phase angle difference between the black drum and the other color drums alters depending on the time of rotation of the black drum in the black mode. Hence, such a problem arises that a color registration error varies (deteriorates) before and after the black mode.